Recently, a technique called “thin provisioning” (for example, see Patent Literature 1) has been actively used in a storage system. Here, the thin provisioning denotes a technique of assigning a virtual volume (or virtual logical volume) to a host computing device. Also, the virtual volume denotes a volume to which an actual data storage area on a physical disk (hereinafter referred to as “page”) is assigned from a pooled logical volume (hereinafter referred to as “pool volume”) at the time when the host computing device writes data in the virtual volume.
Also, as disclosed in Patent Literature 2, depending on the I/O volume from a host computing device for a page assigned to a virtual volume, the page is operated to be moved from a pool volume storing the page to a different pool volume of different performance and cost (rearrangement processing). That is, a storage apparatus records the I/O count for each page comprising a virtual volume (hereinafter execution of this recording is referred to as “I/O monitoring”) and determines a page arrangement destination layer (i.e., a pool volume classified by performance and cost) depending on the I/O count. For example, a page of a larger I/O count is preferentially arranged in a higher layer (i.e., pool volume of high performance and high cost).